This invention relates to a positive type photoresist material.
A conventional positive type photoresist is composed of a mixture of an alkali-soluble novolak type phenol resin and naphthoquinone diazides. However, this type of photoresist is inconvenient in that, since the diazides are unstable to heat, even a portion thereof protected from chemical rays is decomposed when the temperature is raised to a more or less extent so that the quality of the formed image is lowered. This prior art photoresist is also inconvenient in that the diazides need be used at a higher mixing ratio in order that the diazides may act effectively as an inhibitor for inhibiting solubility in a water-soluble alkaline developing solution, and in that the novolak type base phenol resin exhibits considerable absorption of the ultraviolet part of the light rays such that the chemical rays used as the exposure light are not transmitted sufficiently to the bottom of the photoresist layer with the result that it becomes difficult to develop the boundary portion between the photoresist layer and the substrate.
It is known that, when a certain kind of sulfonates is irradiated with chemical rays, sulfonic acids are generated. When a composition containing such sulfonates is irradiated with chemical rays, solubility of the irradiated portion in the alkaline developing solution is increased. For this reason, it has been proposed in the Japanese Laid-open Patent Publication No. 260947/1985 to utilize the sulfonates as the photoresist. Although this prior-art photoresist is improved in thermal stability as compared with the photoresist containing the diazides, it is not entirely satisfactory in storage stability since it is a binary system mixture of the base resin and the photosensitive material, similarly to the conventional photoresist, while it is poor in resolution, which is one of the crucial properties of the photoresist, so that the photoresist cannot be used practically. With the use of a phenol resin as the base resin, transmittance of the photoresist to the ultraviolet part of the light rays is lowered. Similarly, with the use of an ethylenic resin as the base resin, the photoresist exhibits only poor plasma etching resistance.